Diet is determined by socioeconomic factors (e.g. education, occupation, and income) and personal characteristics (e.g. ethnicity, gender, and age). Diet inequalities, whereby diet quality differs systematically across population subgroups, are well-documented. Reducing inequalities has long been prioritised by governmental bodies, yet inequalities persist and effective solutions remain elusive. My thesis aims to further understanding of diet inequalities, particularly focusing on two under-studied groups: food insecure adults and ethnic minority adults. An analysis of UK national nutrition surveys showed substantial social inequalities in diet that largely persisted from 1986 to 2012, though adherence to dietary recommendations improved over time for most population subgroups. Alongside persisting inequalities, food bank usage has risen in the UK. In an online survey, 24% of UK adults reported food insecurity (inability, or perceived inability, to afford a sufficient and nutritious diet). Food insecurity was more prevalent among certain population subgroups, and was associated with poorer diet and health. A thematic analysis of national newspapers revealed public support for the government to address the perceived root causes of food insecurity, through improving welfare support and employment policies. However, existing interventions rely heavily on charitable provision of food for individuals. Analysis of a multi-ethnic sample of Amsterdam residents illustrated that diet quality was not always associated with socioeconomic position in all ethnic groups. Together, these studies show that socioeconomic inequalities in diet persist, but are not inevitable, and demonstrate an intersection between personal characteristics and socioeconomic circumstances in their influence on diet. Socioeconomic disadvantage does not explain all diet inequalities. Community support may be protective against poor diet. Changes to the food and economic systems are likely to be necessary to improve population diet and reduce diet inequalities. Governmental action to make these structural changes is supported by the general public in the case of food insecurity.